I was in my local Gym (LA Fitness) here in Miami working on the heavy bag a few months back, and this big dude came up to me and asked me if I would be interested in an underground fighting tournament. He said you can make around a grand a fight. According to him, he was doing it full time for a year and netted close to 100 grand... tax free of course. In the process, he really fucked himself up though in terms of knee injuries, back and jaw injuries. He said since I had formal training knowledge, the promoter would be able to market me extremely well and I could make a killing. He gave me the promoters contact information and everything. I never did follow through though.
Underground fighting is big down here. This is where Kimbo Slice got his start after all. That guy made a fortune working the backyard circuit, prior to his MMA career.
It's a closed off society. You have to know someone that knows someone in order to be a part of that scene.